1. Field of the Invention
This invention relates to the field of digital communications, and in particular to the communication of a low bit-rate data stream with a high bit-rate data stream, the low bit-rate data stream having an improved bit-error-rate relative to the high bit-rate data stream. This invention allows a higher-reliability bit-stream to be transmitted via an ATSC system with substantial backward-compatibility to existing ATSC receivers.
2. Description of Related Art
The Advanced Television Systems Committee (ATSC) has formulated standards for the transmission of digital television signals. These standards include characteristics of the RF/Transmission subsystem, which is referred to as the VSB (Vestigial Side Band) subsystem of the Digital Transmission Standard. The VSB subsystem randomizes incoming data, then applies forward error correction (FEC) in the form of Reed-Solomon (RS) coding, data interleaving, and trellis coding.
FIG. 1 illustrates a trellis encoder 100 of a conventional VSB subsystem. This encoder 100 consists of a precoder 110, 115 for encoding x1, and a rate ½ feedback convolution encoder 120, 122, 125 for encoding x2, and produces three output bits, z0, z1, z2, for every two input bits, x1, x2. The three output bits z0, z1, z2 are mapped to one of eight analog signal levels, or “symbols” R. The encoding and mapping provide a “gain” to the input bits by improving the likelihood of successful error correction at the receiver. This gain is provided by a combination of the encoding scheme and the mapping scheme.
The encoding of the input bit x2 into output bits z1 and z0 includes redundant information. The value of z1 corresponds directly to x2, while the exclusive-or gate 120 and delay devices 122, 125 provide a value z0 that corresponds to a sequence of x2 values. This redundant information facilitates a higher likelihood of correcting an error that would otherwise affect the decoded value corresponding to x2. Conversely, the exclusive-or gate 110 and delay device 115 encode the value of x1 into a value z2 that corresponds to a sequence of x1 values, and does not provide redundant information that can be used to facilitate the correction of an error.
The mapping of the output bits to the particular symbol R also affects the likelihood of correcting a bit error, by minimizing the effects of a symbol error. For example, the output bit z2 is mapped to the symbol R such that a bit-value of 0 corresponds to the negative symbols (−7, −5, −3, −1), whereas a bit-value of 1 corresponds to the positive symbols (1, 3, 5, 7). Thus, for example, if a z2 bit value of “1” is encoded symbol to 7 (corresponding to z1 and z0 bit values also being 1), the received signal would have to be degraded sufficiently (to at least −1) to cause it to be decoded as a value of 0. The distance required to introduce an error in this example is “8” (7−(−1)). On average, a symbol-error-distance greater than 4 is required to cause a symbol error on the receiver to cause an error in the decoded value of x1. Conversely, the mapping of bit z0 provides no gain, because a change of one symbol level (e.g. to 5) will result in an erroneously decoded bit z0.
Consistent with conventional forward error correcting design techniques, the particular encoding scheme and mapping scheme of FIG. 1 was selected by the ATSC to provide approximately equal likelihoods of error correction for the inputs x1 and x2. The design of the VSB subsystem provides a specified bit error rate (BER) for each input x1, x2 as a function of the signal-to-noise (SNR) ratio. For example, the conventional ATSC terrestrial VSB subsystem has a threshold of visibility (TOV) that corresponds to a segment error rate of 1.93*10−4 at a signal-to-additive-white-Gaussian-noise of 14.9 dB. These characteristics were chosen to provide acceptable performance in the rendering of video content material, and ancillary material, such as TV guides, via terrestrial transmission and reception.
Often, a need exists for communicating information with a substantially lower bit-error-rate than the one provided for communicating video and ancillary information. The current ATSC specifications do not provide for a selectable bit-error-rate quality.